Getting to Grips with a Global
Positioning System (GPS)
Tom Burke
Content
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Introduction
Background
GPS’s & the GPS Systems explained
Limitations of a GPS handheld device
Making your choice – a buying guide
Basic user guide
Useful links - resources
Introduction
Aims
• By the end you should feel you have enough knowledge (or know
where to look for further research) to be in a position to choose &
use the basics a handheld GPS system should you wish.
Objective
• To give an overview of the Global Positioning Satellite System &
how a Handheld Global Positioning System works.
• To provide you with an informative guide to the functions, features &
user interfaces (UI’s) of the various hardware & software options.
• To provide you with a basic user guide for handheld GPS’s.
Background
Global Positioning Satellite System (GPSS):
• The global positioning satellite system was developed by the US
Military during the cold war in order to increase the accuracy of
intercontinental ballistic missiles.
• In order to limit civilian use the they degraded the signal to reduce
accuracy.
• In 1995 the US GPS system became fully operational.
• In May 2000 selective availability was removed to give an average
accuracy of 15m.
• In 2007 the Russian GLONASS system (another GPSS) was
scheduled to become fully operational.
• As of March 2008, the system is not fully available, however it is
maintained and remains partially operational with 16 operational
satellites.
Background cont.
• Aside from the US & Russian systems, two other systems are in the
‘pipe line’.
• The European one which is called GALILEO and was planned for
completion in 2010.
• The other system is being built by China and is called Beidou. It’s
completion was planned for 2008 when it would be providing an
“open” level of service with 10m accuracy.
• However it would appear that both systems are still in their infancy
with a little way to go before completion.
• With Galileo, the second Satellite, of the planned thirty was only
launched in April, they are now predicting an completion date of
2013 with the third satellite scheduled for launch in 2010.
• It would appear that the Chinese system currently only has 4
Satellites (as at Feb 2007) and will continue to be used for testing.
Background cont.
Satellite Based Augmentation Systems (SBAS)
• These systems are designed to increase the accuracy of GPS’s;
typically by a factor of around 5 which equates to a position fix to
3m.
• They work using geostationary* satellites and ground based relay
stations to improve the signal quality from the GPSS.
• Because the system is based on fixed positions, coverage
availability is limited.
• You also require a compatible device which will show in the
specification as WAAS (US) or EGNOS (European) compatible.
*geostationary – in a fixed position above a point on the earth
How the GPS System Works
• GPSS uses a system of
satellites (the US system
uses 24) 11,000 miles above
us in space.
• These are circling along a
number of axis’s around the
globe at 1,000 miles per
hour sending radio signals
back to earth.
• Usually at least 6 satellites
are visible at any one time.
How the GPS System Works cont.
• A GPS Receiver can log on
to 12-14 satellites
simultaneously.
• A GPS needs to log on to at
least 4 satellites and, by
sharing a combination of
distance, time and position
information between the
satellites, gives the user a fix
of latitude, longitude and
elevation.
• This is known as a 3D fix.
Points to Note
• A handheld GPS is an excellent ADDITION to your traditional
navigation methods.
• It will never replace the map & compass
• It will give you a position fix even in hill fog, blizzard or ‘white
out’.
• The service a GPS uses is completely free bar the cost of
replacing batteries.
Limitations of a GPS
• Make & Model
The make and model will govern what your GPS will be capable of.
Obviously you are restricted by what is available but ensure you choose a
make and model that suits your current and future needs.
• Standard Features & Design
The features & design will affect its performance & flexibility.
Memory size, colour screen, electronic compass, barometer & altimeter
can enhance the users experience.
However the compass, barometer and altimeter are only useful if regularly
calibrated & calibrated correctly.
Things like battery type (power) and the antenna type used will affect
signal strength.
• Additional Features
Memory expansion slots for saving data
Included or additionally bought mapping software
PC link-up for upload & download of data and loading maps or new firmware
Limitations cont.
• User Ability
You will only get out of your GPS what you put in. To be able to use
your GPS to its fullest potential you will need to spend time with the
manual / or a guide book to learn how it works. Alternatively you can
book yourself on a dedicated training course.
• Reception Signal & strength
The whole system relies on a good line of sight to the satellites: the
stronger the signal, the more accurate the position fix will be.
Reception is how good the connection to the Satellites are; the more
satellites you can gain reception from them, again the more accurate
your readings.
The available reception will affect the GPS’s signal strength; the
following can affect or block the GPS’s reception & signal strength:
NB. Reception is unaffected by bad weather
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Terrain in your surroundings
Location / Position (in the world)
Battery / Power
Antenna type
Limitations cont.
• Reception cont.
• False Signals
• Atmospheric conditions
• Multipath errors
Terrain in your surroundings
Reception can be affected by Tree cover, Gullies, buildings, your
body and other objects.
Location / Position (in the world)
In some locations the satellite coverage (path routing) is better that
others. If your in a poor area you will not get as good / will struggle
to get a sufficient signal. This could result in a 2D fix (which does
not include elevation) or a False signal.
Battery / Power
In order to obtain a strong signal handshake your GPS sends a
signal to the Satellite. The strength of the signals will determine
which satellites you log onto and this in turn affects the GPS’s
accuracy. When there is sufficient power the signal will run at the
designed maximum but as the power is lost the GPS is unable to
maintain max power which then affects how many satellites it can
reach and logs onto.
Limitations cont.
• Reception cont.
• Antenna type
There are two types of GPS aerial –
quadrifilar helix (right) and patch (left). A
quadrifilar helix aerial is more sensitive and
will generally get a signal where a patch
will struggle.
• False Signals
If a false signal is being received, the speed or location could be
inaccurate. This can be caused by poor satellite geometry. If all the
satellites within your GPS’s line of site are clustered closely together
or are situated in a line away from the antenna, it is more difficult to
calculate positions.
• Atmospheric conditions
This can affect the speed of the GPS signals to the satellite and
cause your handset to do strange things. This could be as a result
of Solar activity in the Ionosphere.
• Multipath errors
A multipath error is where the GPS receives two signals from the
same satellite. This can be caused by a satellite signal being
bounced of rocks or similar in your location.
Buying Guide
1. Zoom In/Zoom Out
4. Find/Options
Whilst viewing the map
screen, press to zoom the
map view in and out. Look
at scale on screen.
On any other page use
these keys to scroll up and
down a list or to move a
slider bar.
Press and release to cycle
through the screen pages
This button can also be
used to ‘quit’ a process
when working within a page.
Press and hold in to toggle
the electronic compass on
and off (certain models
only).
2. Find/Options
Press and hold to show the
waypoint find menu.
Press and release to show
the options menu for a
page.
3. Thumb Stick
(a four way scrolling device and Enter button)
The GPS is designed to be used in the left hand with the
thumb stick being operated with the thumb.
Press down and release to select highlighted options or data
or to confirm on screen messages.
Push up/down/left/right to move through lists, highlight areas,
buttons or icons or to move the map panning arrow on the
map screen.
5. Power/Backlight
Press and hold in to switch
on and off. When GPS is
on, press and release to
switch on and adjust
backlighting, view the date
and time, satellite and
battery status. Move slider
bar on screen to adjust
brightness.
Features
Essential1
Desirable2
Not needed3
My Ideal GPS profile
WAAS enabled
Datebase for at least 500 waypoints
Capacity to store at least 20 routes and 10 tracks
Budget:
Minimum: £________
Exchange data between GPS and PC
Good quality colour screen
Maximum: £_______
Base map with towns and cities included
Memory capacity to store extra points of interest and street mapping
Electronic compass
Electronic altimeter
Elevation profile on screen
Audible proximity alarm
Antenna type
Battery type & number required
Average battery life
User friendly button and menu (Try before you buy)
Fits comfortably in the palm of my hand
Waterproof to IPX7 standard
Cable, case & lanyard included
Downloadable software upgrades
Thinking of the future, will you need the desirable features later?
Other features you might want to consider:
1.
I will not buy a GPS without
these features
2.
These features are nice to
have if the price is right but
not essential
3.
I will not pay extra for these
features as I will never use
them
Large screen & large images on it (important id you wear glasses)
Size of GPS
Weight of GPS
Geocaching features
External antennae interface (useful if using in a car in cities)
Other features?